The sacrificial layers that are currently predominantly used for microfabrication processes such as micromachining are almost exclusively inorganic materials, the most commonly used being silica, i.e., silicon dioxide (SiO2). Aqueous hydrofluoric acid (HF) selectively etches SiO2 in the presence of silicon and silicon nitride, among other materials. HF also etches many other materials, including metal oxides and organic polymers. Although some non-silicon based materials, e.g., titanium and aluminum, can be used as sacrificial layers to be removed by an HF etch, the poor selectivity of this etch beyond oxides limits its usefulness with a wide range of relatively fragile microelectronic materials. Further, the toxicity of HF makes it inconvenient and/or hazardous and presents difficult disposal requirements. HF-free etching solutions for aluminum are available, based on mixtures of acids and oxidants, e.g., concentrated phosphoric and nitric acids, hydrogen peroxide, and acetic acid. However, these acids are incompatible with some fragile materials.
Organic polymers, such as poly(imide), poly(methyl methacrylate) (PMMA) and photoresist, have been used as sacrificial layers for micromachining. The removal of poly(imide) films by reactive ion etching (ME) is compatible with most inorganic materials, but RIE has little selectivity in etching most organic materials. Sacrificial layers of photoresist can be removed by dissolution in acetone, or by thermal degradation, but these removal steps are incompatible with many other organic polymers. Photoresists that are used as sacrificial layers are also limited by their thermal sensitivity, that is, the photoresist film becomes insoluble in acetone after extended exposure to high temperatures. Sacrificial layers of photoresist are, therefore generally restricted to systems including only inorganic materials, and to processes having a minimal exposure to high temperatures.
Conventional pore-forming agents have also been used as a means of producing pores and/or channels in a material. These agents are generally introduced by dispersing the agents into the material in which the pores or channels are to be formed. The pore-forming agent can be removed by washing with a solvent, such an aqueous solvent or inorganic solvent (e.g., salt leaching); volatilization of the agent, such as by sublimation or evaporation; and/or melting. However, the pores and/or channels produced by these methods are typically isotropic, not well-defined and reproducibility is difficult to achieve.
There exists a need for sacrificial materials that can be removed without the need for strong acids and/or organic solvents which are toxic and/or for which disposal is difficult. The sacrificial materials should be processable, that is, can be patterned or etched using techniques known in the art, and result in the formation of well-defined and reproducible features.
Therefore, it is an object of the invention to provide methods for microfabricating composite materials using sacrificial materials that can be removed using aqueous solution thus avoiding the use of strong acids and/or organic solvents and resulting in the formation of well-defined and reproducible features.